1. Field of the Invention
The present invention relates to a method and system for fracturing an earth formation from a wellbore by overpressuring a tubing string and/or casing with pressure gas applied from a surface reservoir and actuator system for releasing gas to initiate and propagate the fracture.
2. Background
In fracturing earth formations from wells to stimulate the production of fluids therefrom, a longstanding problem has been the inability to sustain high pressure and flow of fracturing fluid during the fracture initiation and extension process. In deviated wells, in particular, inadequate fluid pressure and flow conditions at fracture initiation will produce a near wellbore kink in the fracture which will tend to restrict the flow of fluids to or from the wellbore once the fracture has been formed. U.S. Pat. No. 5,074,359, issued Dec. 24, 1991, to Joseph H. Schmidt and assigned to the assignee of the present invention discusses the problem of improper fracture formation from deviated wells, in particular.
Conventional fracturing techniques are also limited by the inability to provide the fracture fluid at sufficiently high flow rate to sustain formation breakdown pressure once fracture initiation or breakdown occurs. In conventional fracturing techniques, the fracture fluid is supplied at a predetermined pressure from surface disposed pumps and friction pressure losses through the pumping system and the wellbore conduits leading to the fractures zone often preclude adequate fracture extension once formation breakdown occurs. One solution to the above-mentioned fracture initiation and extension problem is described and claimed in U.S. Pat. No. 5,271,465 to Schmidt et al. and also assigned to the assignee of the present invention. The '465 patent describes and claims an over-pressured well fracturing method wherein a pressure gas charge is built up in a wellbore conduit or tubing string prior to perforation of the well casing so that, upon firing a perforation gun to perforate the casing, a substantial amount of fluid energy is available for fracture initiation and extension. Alternatively, in accordance with the '465 patent, if the well has already been perforated, a shear disk or other releasable closure member is interposed in the tubing string, preferably at the lower end thereof and a fluid pressure charge is built up in the tubing string to the point of release of the closure member or shear disk to provide a high energy charge of pressure fluid to flow through the perforations into the formation.
The above-described method is particularly advantageous for wells which have relatively large diameter tubing strings which may hold a sufficient charge of pressure gas to provide adequate fracture fluid pressure and flow characteristics. If the well is relatively shallow or the tubing diameter is relatively small, less than about 6.0 inches, for example, a sufficient charge of pressure gas may be provided by precharging a tubing string disposed in an adjacent well and connecting the tubing strings together by suitable conduit means. This method is described in U.S. Pat. No. 5,370,187 to Keith R. Ferguson and Joseph H. Schmidt and also assigned to the assignee of the present invention.
However, an "accumulator" well as described above is not always available closely adjacent to the well from which the over-pressured fracture initiation or extension method is desired to be performed. Moreover, since shallow wells and/or wells with relatively small diameter tubing and/or casing do not have adequate reservoir capacity for the fracture fluid charge, a surface disposed reservoir is desirable. However, heretofore the fluid pressure and volume requirements for initiating fractures have presented certain problems with respect to providing a suitable operating method and a structurally adequate reservoir and flow control mechanism for releasing the high pressure fluid to flow into and through the well to initiate or extend a fracture.